It is often desirable to make particles of partially cured or fully cured aminoplast and phenoplast resins such as melamine-formaldehyde resin, urea-formaldehyde resin and phenol-formaldehyde resin, commonly referred to respectively as melamine, urea and phenolic resins. These particles can be used as fillers either with or without inorganic fillers, and as molding powders. A particularly important use of such particulate materials, especially partially cured melamine resin particles, is in the manufacture of abrasion resistant aesthetic surface layer decorative laminates according to the O'Dell et al U.S. Pat. No. 5,344,704, which surface layer also desirably contains inorganic grit having a Moh hardness desirably greater than 6 or 7 in order to provide enhanced abrasion resistance.
Water-based thermosettable resins, e.g. aminoplast and phenoplast resins such as melamine resin, urea resin, and phenolic resin, have a characteristic which is sometimes known as "water tolerance". Thus, these aminoplast and phenoplast resins in the uncured state are water soluble, but when additional water is added beyond the so-called "water tolerance level", these resins begin to precipitate and form an oil-in-water emulsion or suspension, depending on the degree of cure. This known phenomenon is something which handlers of aminoplast and phenoplast resins have known of and have usually tried to avoid because it "ruined" the resin.
Thus, if sufficient water is added to such an aqueous thermosettable resin solution so as to exceed the water tolerance level and form an oil-in-water emulsion of the resin, and one then tries to advance the cure of the resin to partial or complete cure, the particles tend to coagulate and become agglomerated; and as the resin cures it forms large, hard rock-sized chunks of cured or partially cured resin which, unless these chunks can be ground to powder, are substantially useless.
A few workers in the field have attempted to use "water tolerance" to make thermosettable resin powders. Thus, the Renner et al U.S. Pat. No. 3,428,607 discloses the manufacture of melamine resin powders of less than 1 .mu.m particle size by slowly adding the aqueous resin or resin precursor to water containing a protective colloid such as starch, gelatin, glue, gum tragacanth, agar-agar, carboxymethyl celluloses, alkaline metal alginates and water soluble polymers such as polyvinyl alcohol, polyvinyl pyrrolidone, alkali metal salts of polyacrylic acids, etc. at a concentration between 0.01% and 10%, the formation of the particles being carried out at a pH of 6-8 and preferably at the boiling temperature of the liquid. A very similar process is disclosed in the Tsubakimoto et al U.S. Pat. No. 3,945,980, in which the resin is a benzoguanamine resin. Insofar as is known, neither these methods nor the resultant products have achieved any degree of commercial success, and thus these methods are believed to have become abandoned.
In O'Dell et al, Ser. No. 08/516,738 now U.S. Pat. No. 5,728,797, an improved process is disclosed for obtaining phenoplast and aminoplast resins in particulate form. Insofar as is known, however, thermosettable resin powders in the form of shells encapsulating preformed organic or inorganic cores, e.g. grit, have not been proposed or produced. These core-in-shell powders would not only be advantageous in the preparation of molded resin products and intermediate-stage or "green" moldings in the production of ceramics, but would especially be useful in the manufacture of laminates in general and especially according to O'Dell et al U.S. Pat. No. 5,344,704 and O'Dell et al U.S. Pat. No. 5,545,476 to protect pressing plate dies against wear caused by the inorganic grit, and in the manufacture of particulate resin containing paper on a paper-making machine.